Attempts to develop an airborne systems for laser image capturing, or laser viewing, are known, and they generally comprise a laser light source for illuminating a target, and means sensitive in the laser light wave-length, such as a focal plan array, for sensing the light reflected from the target (hereinafter, if not specifically otherwise stated, when the term “laser viewing system” is used, it should be noted that it refers also to a “laser capturing system”, and vice versa). In some suitable conditions, and generally at dark, laser systems enable the obtaining of target images at ranges slightly larger than can be obtained by equivalent thermal viewing systems.
Two main factors affect the operation of long range laser viewing systems: the conditions of the medium and the stability of the line of sight. In ground laser viewing systems, the laser beam performs a round-trip to the target and back in a generally “unfriendly” medium, which causes an attenuation of the laser signal due to, for example, scattering of the ray, and/or air turbulence. In an airborne system, which operates at high elevations the effects of the medium conditions are significantly reduced because the ray passes mostly through a clear air, while the effect of the stability of the line of sight becomes much more dominant. When the line of sight between the laser source within the airborne system and the target is not sufficiently stable, the sensed image is smeared.
The prior art has offered two main solutions for reducing smearing of the image due to vibrations of the line of sight, as follows:                1. Imaging by one pulse: In this approach, both the laser transmitter and the laser focal plane array are directed towards the target, and then the whole target is illuminated by means of a laser beam made of a single laser pulse in the range of several nanoseconds. In order to illuminate the whole target, the laser beam (which is originally very narrow when emitted from the laser diode), is optically expanded by means of suitable optics before leaving the apparatus (as otherwise the laser beam would illuminate only a small portion of the target). The focal plan array is adapted to receive an image of the whole target in one capturing cycle. The main drawback of this approach is that the expansion of the beam, and the need to obtain an image of the target in one pulse cycle, which is a part of this approach, significantly reduces the amount of photons per unit area that hit any portion of the target, and therefore the maximum effective range of the system is relatively low.        2. Imaging by series of pulses—photon accumulation approach: As in the previous prior art approach, in this approach, the laser source illuminates a whole scene which includes the target by means of a series of laser pulses. The laser beam, after being collimated, is maintained directed towards the target during the accumulative time of all the pulses, while the line of sight to the target is kept as stable as possible by means of gimbals and a compensation unit that provides a motion compensation signal to the gimbals during the period of the pulse transmission and image capturing. The focal plan array is activated during a “window” period in which a reflected pulse is expected, and accumulates the sensed radiation in corresponding pixels, until accumulation of enough photons provides a reasonable signal to noise ratio. The main drawback of this approach is the need to keep a stable line of sight to the target during the transmission of all the series of pulses, and to measure and compensate for very fast and small vibrations of the line of sight. This is generally a very complicated task, particularly in view of the movement and vibrations of the aircraft, resulting in a smeared and/or corrupted image.        
It is an object of the present invention to increase the effective range of an airborne laser image capturing system.
It is another object of the present invention to provide means for overcoming the drawback of smeared and low quality images that can be obtained at long ranges, due to a non-stabilized line of sight to the target.
Other objects and advantages of the invention will become apparent as the description proceeds.